The present invention relates to an apparatus for loading or unloading a workpiece, and more particularly, to an apparatus for loading or unloading a workpiece on or from a press die or a jig.
An apparatus for loading or unloading a workpiece has been installed in a press in order to load or unload a workpiece on or from a press die or jig. There has been known such a loading or unloading apparatus as the apparatus can feed a workpiece in not only the horizontal direction but also in the vertical direction. Two kinds of such apparatuses are known. The first kind of apparatus is of a type in which a power source which is common to a manufacturing machine such as a press and to a loading or unloading apparatus, is provided to obviate the temporary stoppage of operation of the manufacturing machine. The second kind of apparatus is of a type in which power sources are separately provided on a manufacturing machine for deeply drawing a workpiece, and a loading or unloading apparatus in order to obtain an automatic operation. However, according to the second kind of apparatus, a temporary stoppage of operation of the manufacturing machine cannot be obviated.
According to the above-mentioned first kind of apparatus, a workpiece holder performs a work motion and further, a return motion when the apparatus returns to a previous position in order to commence to load or unload a workpiece on a manufacturing machine. FIG. 21 illustrates the work motion and the return motion of the workpiece holder 1 according to a prior art. To start or stop the operation of the apparatus, the apparatus has to be located at one of both ends of the work motion or the return motion. If the apparatus is started or stopped on the way of the work or return motion, the moving speed of the workpiece holder 1 is high, a forced actuation for starting or stopping the operation impairs the endurance of the loading or unloading apparatus. To shorten the time when the workpiece holder 1 does not hit a manufacturing machine, it is necessary to minimize the upward and downward stroke of the loading or unloading apparatus. If the upward and downward stroke of the loading or unloading apparatus is made small, a lifter has to be separately installed for compensating the upward and downward stroke of the loading or unloading apparatus.
On the other hand, according to the above-mentioned second kind of apparatus, the perpendicular position of the workpiece holder 1 before or after the workpiece holder 1 starts or finishes the operation, is relatively positioned at a high point because the working motion is same as the return motion, as shown in FIG. 22. The reference h designates the distance in the perpendicular direction where the workpiece 50 is moved by the work motion or the return motion. The reference H designates a stroke where the press repeatedly moves in the perpendicular direction. As apparent from FIG. 22, the high point of the stroke h is close to the high point of the stroke H. Hence, when a workpiece 50 is loaded on a press die or a jig, a manufacturing machine must be temporarily stopped at its top dead point in order to obviate such as problem as the manufacturing machine strikes against the loading or unloading apparatus. FIG. 24 shows a diagram which illustrates the relation between the press stroke and the crank angle of the press. In FIG. 23, a finished product is unloaded during a time between points A and C. A workpiece is loaded during a time between points B and D. The reference TDP designates a top dead point of the manufacturing machine, and the reference BDP designates a bottom dead point thereof. In FIG. 24, the manufacturing machine stops during a time period between crank angles CA.sub.1 and CA.sub.2 while the apparatus maintains the position of the top dead point. Thus, the manufacturing machine has to temporarily stop at the tip dead point, and hence, the manufacturing machine intermittently operates. This results in decrease in the manufacturing efficiency and further in the occurrence of the wear of parts employed in the loading or unloading apparatus.